In a medium bowl, combine the chicken, 1 tablespoon sherry, the garlic, 2 teaspoons cornstarch, 1/2 teaspoon salt and 1/4 teaspoon pepper. Stir in 2 teaspoons oil.
In a small bowl, combine the chicken broth and the remaining 1 tablespoon sherry. Stir in the remaining 1/2 teaspoon cornstarch until dissolved.
Heat a 14-inch, flat-bottomed wok over high heat until a bead of water vaporizes upon contact. Swirl in 1 tablespoon oil. Add the chicken mixture, spread in a single layer and cook undisturbed for 1 minute. Using a metal spatula, stir-fry until the chicken begins to brown, about 1 minute; transfer to a plate.
Lower the heat to medium and swirl in the remaining 1 tablespoon oil. Add the onion and bell pepper; season with salt and stir-fry until softened, about 1 minute. Add the paprika, return the chicken and any juices to the wok, increase the heat to high and stir-fry until just combined. Swirl in the broth mixture and stir-fry until the chicken is just cooked through, 1 to 2 minutes. Stir in the sour cream and parsley.